A US space agency (Nasa) mission to study auroras - the northern lights - has blasted off from Cape Canaveral in Florida, after it was delayed by wind.

The Themis mission, comprising five identical probes, aims to gain new insights into the colourful displays in high-latitude skies.

In particular, scientists want to understand what triggers a sudden brightening of the lights.

The rocket carrying the probes launched at 1801 EST (2301 GMT).

Power in numbers

Auroras have their origin in the vast clouds of charged particles that billow away from the Sun.

When these are accelerated by the Earth's magnetic field into the upper atmosphere, they will collide with, and excite, gas molecules, which then emit light when they return to a more relaxed state.

From time to time, this green band of light will brighten, then break into many bands that dance rapidly and turn red, purple and white.

These events are called auroral substorms. One of the main aims of the Themis (Time History of Events and Macroscale Interactions During Substorms) mission is to determine how these are initiated.

"A substorm starts from a single point in space and progresses past the Moon's orbit within minutes, so a single satellite cannot identify the substorm origin," said Vassilis Angelopoulos, the Themis principal investigator at the University of California, Berkeley.

Above and below

Three events in the tail of the Earth's magnetic field are associated with the onset of substorms: current disruption, auroral eruption and magnetic reconnection.

But scientists are divided about the order in which these occur.

"The first spacecraft will identify the start of the substorm; where the disturbance first occurs in the Earth's magnetic field, and the other spacecraft will be used to determine how fast and how quickly that disturbance spreads towards other locations," Themis project scientist Dr David Sibeck told BBC News.

The mission only needs four spacecraft to complete the task; the fifth probe is essentially a spare that will provide additional data.

The satellites will magnetically map the North American continent every four days for approximately 15 hours each time.

Simultaneously, 20 ground stations in Alaska and Canada with automated, all-sky cameras and magnetometers will document the auroras and space currents from Earth.

Exploration team

The mission, though, is far more than a simple quest to understand pretty lights in the sky.

When the Sun is particularly active, a series of 10 or more substorms can occur in rapid succession.

"During some of these very intense auroras, there could be disruption to power grids on the ground as well as disrupting communication with satellites; so it's important to be able to predict when these things will happen," Dr Tai Phan, a mission scientist, said.

The quintuplets join a fleet of Sun-Earth connection explorers already in orbit - four from the European Space Agency's (Esa) Cluster mission and two from the Chinese space agency (CNSA) Double Star mission, which is run in collaboration with Esa.

All the spacecrafts' efforts are highly complementary, and several European research institutes will take an active part in the Themis venture.

"This is the first time in history of space physics that such a high number of scientific satellites are in operation simultaneously," observed Esa's Philippe Escoubet, the Double Star and Cluster project scientist.

"It represents an unprecedented opportunity to study the global solar-magnetospheric environment and the physical processes involved."

Logged

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